Magmatic volatile budgets of the 2014 Tavurvur eruption at Rabaul Caldera, Papua New Guinea

Melina Höhn, Brendan T. McCormick Kilbride, Margaret E. Hartley, Mike Burton, John Dikaung, Ben Esse, Ima Itikarai, Kila Mulina, Steve Saunders, Mikhail Sindang, Isabelle A. Taylor, EIMF -

Rabaul is a caldera volcano on New Britain Island, Papua New Guinea, whose active cone Tavurvur ranks seventh globally for long-term mean SO2 and CO2 emissions. It is unknown why Rabaul is such a strong emitter of volcanic gases. Magma mixing between basaltic and dacitic magmas is envisioned to play a fundamental role in driving eruptions at Rabaul, but the compositions of mafic recharge magmas, their volatile contents, and their contribution to ongoing gas emissions are poorly constrained. Rabaul’s recent eruptive history includes the 1994 twin eruption, a VEI 4 eruption in 2006, and a prolonged phase of strong SO2 degassing from late 2006 to 2009 prior to the most recent eruption in 2014. We analysed 84 plagioclase-, clinopyroxene- and olivine-hosted melt inclusions from 2014 Tavurvur volcanic bombs for major, trace and volatile elements (H2O, CO2, SO2, Cl, F), corrected for post-entrapment crystallisation, and combined these data with satellite-derived SO2 retrievals (OMPS, OMI, IASI). Olivine- and high-Mg# clinopyroxene-hosted melt inclusions record basaltic compositions likely representing the mafic recharge magmas of the volcanic system. Low‑An plagioclase- and low-Mg# clinopyroxene-hosted melt inclusions are andesitic to dacitic in composition which closely match dacites erupted in caldera-forming eruptions at Rabaul (e.g. the 1400 years B.P. Rabaul Pyroclastics event) and likely record the composition of magma resident in Rabaul’s shallow plumbing system. Reverse fractional crystallization and trace element constraints imply primary volatile contents of ~1.1–1.9 wt% H2O, 1400–2600 ppm SO2, ~392 ppm Cl and ~59 ppm F, indicating Rabaul magmas are not anomalously volatile-rich within the global range of subduction zone magmas. Our analyses of satellite data yield a best estimate of SO2 mass of ~37 kt for the 2014 eruption. Petrological estimates derived from our melt inclusion analyses give ~110 kt. We attribute the modest 2014 SO2 release to low erupted magma volumes and extensive pre-eruptive degassing of the shallow reservoir between 2006–2010, and note that volatile-rich mafic magma was volumetrically minor in the erupted material. Our results reconcile melt-inclusion and satellite records and constrain the roles of magma mixing and prior degassing in governing Rabaul’s gas output.